Take the gun out of the holster, then warm the tight spots (evenly on both sides of the holster) with a hair dryer, then holster and unholster until it loosens up a bit. When I say warm, I mean warm to the touch. If you heat the holster too much, it'll get floppy and then you're screwed (firsthand experience). Paitence in this process is key.
Better to die on you feet than to live on your knees.